One in every thirteen babies born each year in the United States is born too early, too small, or both. Low-birth-weight infants account for 70% of all neonatal deaths. If the number of premature and low-birth-weight infants could be reduced, many babies would be saved. Unfortunately, the mechanisms leading to the initiation of labor in primates and the hormonal factors controlling fetal development are still poorly understood. I will use chronically catherized rhesus monkey fetuses and double isotope radiotracer infusion techniques, to study endocrine functions of the placenta as they relate to modulation of the fetal endocrine milieu and the control of parturition. This revised grant is focused on how the placenta acts as an interface between the mother and fetus with respect to steroidal exchange between the two circulations. The interactions studied are limited only to those that involve the placenta and adrenal glands and that have significance with respect to parturition. Specifically I will: 1) Determine how placental secretion of estrogens is influenced by fetal adrenal precursors versus maternal adrenal precursors, and 2) determine if circadian rhythams exists in fetal and maternal adrenal steroid production rates, and if maternal adrenal function modulates fetal adrenal activity (and thus the timing of labor) by transplacental passage of maternal cortisol into the fetal circulation. The long-term goals of this grant are to: 1) Determine how the placenta controls hormonal interactions between mother and fetus, and determine whether transplacental passage of maternal hormones modulate development of the fetal endocrine system and influence the timing of labor, and 2) continue to improve our intrauterine surgical techniques and the chronically catherized rhesus monkey fetal preparation so that human health can be improved by studying a primate fetus in utero.